(1) Field of the Invention
The present invention relates to an electrochromic device, and to electrochromic substances.
Electrochromic devices are already known, for example from D. Theis in Ullmann's Encyclopaedia of Industrial Chemistry, vol. A 8, p. 622, Verlag Chemie 1987 and WO-A 94/23333. A distinction is made between two basic types:
(2) Description of Related Art
Type 1: full-area electrochromic devices;
Type 2: electrochromic display devices having structured electrodes.
Type 1 is used, for example, in electrically darkenable window panes or electrically dimmable automobile mirrors. Such devices are disclosed, for example, in U.S. Pat. No. 4,902,108.
Type 2 is used in segment and matrix displays. Such display devices are proposed, for example, in DE-A 196 31 728. Devices of this type can be observed transmissively or, in the case of reflection, reflectively.
WO-A 94/23333 compares electrochromic materials having different constructions, but these are not used as display devices:                Construction a: the electrochromic substances are in the form of a fixed film or layer on the electrodes (Ullmann, see above).        Construction b: the electrochromic substances are deposited on the electrodes as a layer by the redox process (Ullmann, see above).        Construction c: the electrochromic substances remain permanently in solution.        
For construction a), the best-known electrochromic material is the tungsten oxide/palladium hydride pair.
For construction b), viologens have been described as electrochromic substances. These devices are not self-erasing, i.e. the image produced remains after the current has been switched off and can only be erased again by reversing the voltage. Such devices are not particularly stable and do not allow a large number of switching cycles.
In addition, such cells constructed using tungsten oxide/palladium hydride in particular cannot be operated in transmitted light, but only reflectively, owing to light scattering at these electrochromic layers.
Elektrokhimiya 13, 32–37 (1977), U.S. Pat. Nos. 4,902,108 and 5,140,455 disclose an electrochromic system of the latter construction c). An electrochromic cell which is constructed from glass plates with a conductive coating comprises a solution of a pair of electrochromic substances in an inert solvent.
The pair of electrochromic substances used is one electrochemically reversibly reducible substance and one reversibly oxidizable substance. Both substances are colourless or only weakly coloured in the ground state. Under the action of an electric voltage, one substance is reduced and the other oxidized, both becoming coloured. When the voltage is switched off, the ground state re-forms in the case of both substances, resulting in disappearance or fading of the colour.

U.S. Pat. No. 4,902,108 discloses that suitable pairs of redox substances are those in which the reducible substance has at least two chemically reversible reduction waves in the cyclic voltammogram and the oxidizable substance correspondingly has at least two chemically reversible oxidation waves.
Various applications have been described for electrochromic cells of construction c). For example, they can be formed as automobile rear-view mirrors which can be darkened during night driving by application of a voltage and thus prevent dazzling by the headlamps of following vehicles (U.S. Pat. Nos. 3,280,701, 4,902,108 and EP-A 0 435 689). Furthermore, such cells can also be employed in window panes or automobile sunroofs, where they darken the sunlight after application of a voltage. Likewise described is the use of such devices as electrochromic display devices, for example in segment or matrix displays having structured electrodes (DE-A 196 31 728).
The electrochromic cells normally consist of a pair of glass plates, of which, in the case of the automobile mirror, one is mirrored. One side of these plates is coated over its surface with a light-transparent, electroconductive layer, for example indium-tin oxide (ITO), and in the case of display devices this conductive coating is divided into electrically separated segments provided with individual contacts. These plates are used to construct a cell by joining them by means of a sealing ring with their electroconductively coated sides facing one another to form a cell. This cell is filled with an electrochromic liquid via an opening, and the cell is tightly sealed. The two plates are connected to a voltage source via the ITO layers.
For many applications it is important to be able to adapt the colour of the electrochromic device in the energized state to the particular requirements. A colour which is as neutral as possible is often desired, for example a grey colour. Although dipyridinium salts (viologens) and dihydrophenazines are proven electrochromic compounds, mixing them results in a greenish blue colour (e.g. U.S. Pat. No. 4,902,108, WO 97/30134). WO 98/44384 discloses derivatives of dipyridinium salts and dihydrophenazines whose colours are different from the parent compounds. By appropriate mixing of up to 5 electrochromic compounds of this type, a preselected colour can be achieved, including, for example, a grey colour. However, complex mixtures of this type are difficult to balance, especially when the electrochromic device is to be operated at different voltages and the colour is to be independent from the voltage. Furthermore, the dipyridinium salt and dihydrophenazine derivatives disclosed in WO 98/44384 are not always readily obtainable.
Accordingly, there was a need for an electrochromic compound which harmonizes well with dipyridinium salts and dihydrophenazines in terms of redox behaviour, provides the missing red component in admixture with dipyridinium salts and dihydrophenazines in the solvents customary for electrochromic cells, and is readily obtainable synthetically.